1. Field of the Invention
The present invention relates to a digital servo system. More specifically, the present invention relates to a digital servo system which is provided with a digital filter in a servo loop, and a video tape recorder utilizing such a digital servo system.
2. Description of the Related Art
A digital servo system is used for control of a cylinder motor used in a video tape recorder, for example. In the prior art, in order to provide a high servo gain at a low frequency region in a normal state, in general, a digital filter is inserted in a servo loop as a lag filter.
If the digital filter is inserted in the servo loop, the servo gain at the low frequency region in the normal stage is caused to be at a high level and thus stability increases; however, there occurs a problem of a phase pull-in time at a start of rotation, that is, a step response, is increased.
In order to solve such a problem, a technique is used wherein the digital filter is not inserted in the servo loop at the start of rotation, but is inserted after the rotation approaches the normal state. However, in such a case, there is a further problem in that a discontinuity occurs in a phase error signal, and thus a discontinuity arises in a control signal by which the rotation of the rotation body is controlled, both before and after the insertion of the filter. Therefore, a variation or fluctuation occurs in the rotation of the motor.
Furthermore, a digital filter has been considered having a dynamic range by which the step response and the resistance for the disturbance can be respectively comprised. However, in such a case, there is a further problem in that it is impossible to fully utilize the digital filter as a lag filter.